The present invention relates to a temperature compensated filter comprising a body of dielectric material having at least one transmission line resonator formed therein.
A dielectric filter is disclosed in European patent application EP-A-0,401,839 and corresponding U.S. Pat. No. 5,103,197, comprising a body of dielectric material which has upper and lower surfaces, two side surfaces, two end surfaces, and at least one hole extending from said upper surface towards said lower surface, an electrically conductive layer covering major portions of the lower surface, one side face, both end faces and the surface of said at least one hole so as to form said at least one transmission line resonator.
The properties required from the dielectric material are a high proportional dielectrical coefficient .epsilon.r and a small dissipation coefficient. The difficulty with this is that although materials with sufficiently high dielectric coefficients (about 8-100) and low temperature dependence, are available on the market they are relatively expensive and difficult to procure. Relatively good .epsilon.r values and a low temperature dependence of frequency can be obtained with ceramic compounds, for example, but the dissipation coefficients generally increase in these compounds.
The purpose of the present invention is to arrange, by using comparatively simple means, the temperature compensation of the frequency of a dielectric filter in which the material of the dielectric body can be chosen relatively freely on the basis of price and an advantageous dissipation coefficient.
According to the present invention a dielectric filter having the features mentioned in the opening paragraph above is characterized in that a capacitor is coupled to the transmission line resonator for tuning the filter and having a temperature coefficient of frequency opposite that of the dielectric body.
The capacitor itself forms part of the resonance circuit the frequency of which varies with temperature in the opposite sense to the frequency variation of the filter. Since the capacitor is coupled to the "main" transmission line resonator it has the effect of temperature compensating the filter.
Suitably, the filter may have a structure in accordance with that disclosed and claimed in the aforementioned European patent application and the corresponding US patent.
The capacitor may be a so-called chip capacitor which is attached to the dielectric body adjacent the hole therein, preferably on a side surface where the conductive layer is not present.
In a preferred embodiment the capacitor has one terminal electrically coupled to the electrically conductive layer, preferably through a conductive strip provided on the side surface of the dielectric body where the conductive layer is not present. The other terminal of the capacitor may also be coupled to a further conductive strip on the same side face.